This invention relates to a method of papermaking, which renders it possible to make paper with a high filler content (&gt;15%), a high retention of the filler, and substantially improved mechanical and optical properties.
In order to reduce papermaking costs, it is tried at present to increase the content of mineral fillers in the paper. Examples of such fillers also are kaolin, different types of calcium carbonate and talcum. The fillers improve the opacity and printability properties of the paper. In addition, for various reasons it is often desired to make paper with bentonite, titanium oxide, wollastonite, glass fibres, zinc pigment etc. The present invention comprises either an addition of a filler type or of mixtures of different fillers and pigment types.
Addition of fillers give rise to the technical problem that they, to an unsatisfactory degree, deteriorate the strength properties of the wet web as well as the dry paper. Traditionally different types of starches were typically added, into the stock, into the size press, or by a spray method in order to improve the strength properties. Cationic or amphoteric starches normally are used as additives to the stock at present. By derivatization of the starch it is sought to obtain a good retention of filler, pigment and other fine material on the wire and also to obtain maximum dry strength effect of the additives. It is important in this connection that the starch derivative have good affinity to fibres and fillers in the stock. This is normally achieved by cationization of the starch so that it is adsorbed to the negatively charged fibres. It is generally known that a high retention effect in a papermaking machine can be obtained by consecutively adding to the stock both a cationic starch and an anionic polymer, for example polyacrylamide. The synergistic effect is due to the fact that the two oppositely charged polymers interact with each other, although the mechanism in detail is unknown.
When it is desired that large amounts (&gt;2%) of starch be adsorbed to fibres and fillers, it is favourable to use a relatively low-substituted cationic starch (D.S.=substitution degree of cationic groups). D.S.&lt;0.03. There is in fact an optimum charge density of the starch corresponding to a maximum adsorption to a given stock under given chemical conditions. When such a low-substituted cationic starch is used in combination with an anionic high-polymer, however, in most cases an inferior retention effect obtained is inferior to that obtained if a high-charged starch type is chosen. While this can be counteracted to a certain degree by choosing a high-charged anionic high-polymer, but in most of the cases this does not help.